usb: cdc-wdm: Fix race between autosuspend and reading from the device

While an available response is read the device must not
be autosuspended. This requires a flag dedicated to that
purpose.

Signed-off-by: Oliver Neukum <neukum@b1-systems.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>